Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A first known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen ions migrate from a cathode to combine with hydrogen, forming water at the anode; such fuel cells are referred to in the art as “solid-oxide” fuel cells (referred to herein as SOFC). A second known class of fuel cells includes a membrane through which hydrogen ions (protons) migrate from an anode to combine with oxygen, forming water at the cathode; such fuel cells are referred to in the art as proton exchange membrane fuel cells (referred to herein as PEM). PEM are also known as polymer electrolyte membrane fuel cells. In both classes of fuel cells, electrons flow through an external circuit between the electrodes, doing electrical work in a load in the circuit.
In the prior art, an SOFC is readily fueled by “reformate” gas, which is the effluent from a catalytic hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C. An SOFC can use fuel gas containing CO with the H2, the CO being oxidized to CO2, whereas a PEM cannot oxidize CO; in fact, CO is a poison to the catalysts in the PEM stack. Thus CO must be removed from a reformate stream for PEM use (to near zero levels).
PEM and SOFC systems are being developed in the fuel cell art for use in transportation applications, for both primary motive power and for on-board power generation, as well as for stationary applications such as building heating and water heating. A PEM operates at about 100° C. and has a very short start-up period. The waste heat from a PEM is not of high quality because of the relatively low operating temperature. Conversely, an SOFC operates at about 800° C. and has a relatively long start-up period, but the waste heat from an SOFC is of higher quality because of the higher operating temperature. Both SOFC and PEM systems need about 10% of their power capability to power the balance of plant loads, thus affecting their net electrical capabilities.
What is needed in the art is a means for combining the advantages and minimizing the drawbacks of an SOFC and a PEM in an integrated system.
It is a principal object of the present invention to integrate an SOFC system and a PEM system as a novel hybrid system, and to fuel both from a common hydrocarbon reformer.